Ink jet inks for industrial use have been extensively developed so that the inks can be used in direct printing on recording media that hardly absorb inks, e.g., recording media having poor water absorbability such as resin- or clay-coated paper, recording media coated with resin components, or resin films themselves, as well as on paper and cloth having ink absorbability.
Examples of ink jet inks satisfying such requirements include solvent ink jet inks containing organic solvents as vehicles and ultraviolet curable ink jet inks containing photopolymerizable monomers as their main components. In the solvent ink jet inks, the solvents are evaporated, and most of the solvent components dissipate in the air. This causes generation of a large amount of volatile organic compounds (VOC), which is nowadays a socially disputed issue. In addition, odors and adverse safety effects on workers are concerned, and facilities, such as sufficient ventilation systems, for overcoming such concerns are required in use of the ink. In contrast, ultraviolet curable ink jet inks can be cured immediately after printing and thus are substantially free of VOC. Some of the ultraviolet curable ink jet inks, however, have skin sensitization potential due to monomers contained therein. In addition, ink jet printers are required to be equipped with expensive ultraviolet light sources. Accordingly, fields to which the ultraviolet curable ink jet inks can be applied are inevitably limited. Furthermore, upon printing on glossy sheets, the glossiness is considerably lost at areas where the inks are applied, which makes it difficult to yield an image having high quality.
In consideration of these circumstances, approaches using aqueous ink jet inks have been extensively developed for printing on recording media having poor absorbability, as well as on recording media having ink absorbability, such as cloth and paper. The aqueous ink jet inks (hereinafter, referred to as aqueous inks or simply inks), primarily composed of water, have a low environmental load and have been widely used at home. Unfortunately, recording media having poor absorbability, such as coated paper and resin films, have low surface energy in many cases and do not absorb typical aqueous inks. Consequently, ink droplets landed on such a recording medium are repelled to generate pin hole in an image or unevenness in a solid portion, resulting in a decrease in image quality. Another disadvantage is that the recording medium has no aqueus ink absorbency; this prolongs drying time and causes deposition of unabsorbed coloring materials on the recording medium, resulting in a decrease in image durability such as friction resistance.
A method of preventing repelling of an aqueous ink by adding a surfactant or an aqueous organic solvent having a low surface tension to the ink improves wettability of the aqueous ink to a nonabsorbent medium and thereby prevents occurrence of repelling to some extent. However, addition of an excess amount of a surfactant causes precipitation of the surfactant after the ink dries, resulting in a decrease in image glossiness. Since the surfactant is localized at the interface between the recording medium and ink, the formed image layer does not stick to the recording medium, in spite of prevented repelling. As a result, adhesion of the formed image to the recording medium decreases to cause a reduction in image durability. Although the use of an organic solvent having low surface tension does not cause precipitation, it does not increase the friction resistance. In addition, many of the organic solvents having low surface tensions have high boiling points, causing poor ink drying characteristics.
A measure for solving these disadvantages involves the addition of a solvent can be absorbed into a polyvinyl chloride sheet to improve friction resistance (e.g., see PTL 1). In particular, PTL 1 claims that an ink containing a β-alkoxypropionamide exhibits excellent glossiness and friction resistance and is also effective for resolving the ink repelling issue. Unfortunately, the results of investigation by the present inventors show that although the method described in PTL 1 prevents weak repelling to some extent, effects on preventing strong repelling and pin holes are still low. Consequently, the method described in PTL 1 is still insufficient for preventing formation of defected images (e.g., unevenness or pin holes).
Another disclosed approach to improve image quality is to adjust the solubility parameter of a solvent in an aqueous ink to approximately the solubility parameter of a plasticizer, such as polyvinyl chloride, contained in a nonabsorbent base material (e.g., see PTL 2). Unfortunately, the results of investigation by the present inventors show that although a certain improvement is observed in a region to which a small amount of ink droplets is applied, repelling and pin holes occur in a region, such as solid printing, where a large amount of ink droplets is applied and that the resulting image has a large difference in glossiness between the white surface portion of the base material and the image portion to give discomfort feeling in visual observation.
Methods are also disclosed that involve addition of surfactants to aqueous inks to decrease repellency of the aqueous inks on nonabsorbent base materials (e.g., see PTLs 3 to 6). Unfortunately, the results of investigation by the present inventors show that the wettability of an aqueous ink to a nonabsorbent recording medium is improved by the addition of a surfactant, but an improvement in image quality, in particular, unevenness and pin holes in solid portions, is still insufficient. On the contrary, the addition of a surfactant further decreases image quality in some cases.
Thus, in the case of recording on a nonabsorbent recording medium, in particular, on a vinyl chloride sheet, with an aqueous ink, it is significantly difficult to form an image having high fineness comparable to that of an image formed with a solvent ink.